The Joomla 2.5.* version of the Web Links Component is missing a class for the descriptions in the list view of a category. This means you can't control the look of the description using css. To fix this you can do a simple template override.

First of all, locate the file

components/com_weblinks/views/category/tmpl/default_items.php

Create in your template, in the HTML directory the following subdirectory structure:

yourtemplate html com_weblinks
category

and copy there the above file. Now open the file, and locate this line (around line 131, depending on your exact version: